Various communications schemes have been used to increase data throughput and to decrease data error rates as well as to generally improve the performance of communications channels. As an example, frequency division multiple access (“FDMA”) employs multiple data streams that are assigned to specific channels disposed at different frequencies of the transmission band. Alternatively, time division multiple access (“TDMA”) uses multiple data streams that are assigned to different timeslots in a single frequency of the transmission band. FDMA and TDMA are quite limited in the number of users and/or the data rates that can be supported for a given transmission band.
In many communication architectures, code division multiple access (CDMA) has supplanted FDMA and TDMA. CDMA is a form of spread spectrum communications that enables multiple data streams or channels to share a single transmission band at the same time. The CDMA format is akin to a cocktail party in which multiple pairs of people are conversing with one another at the same time in the same room. Ordinarily, it is very difficult for one party in a conversation to hear the other party if many conversations occur simultaneously. For example, if one pair of speakers is excessively loud, their conversation will drown out the other conversations. Moreover, when different pairs of people are speaking in the same language, the dialogue from one conversation may bleed into other conversations of the same language, causing miscommunication. In general, the cumulative background noise from all the other conversations makes it harder for one party to hear the other party speaking. It is therefore desirable to find a way for everyone to communicate at the same time so that the conversation between each pair, i.e., their “signal”, is clear while the “noise” from the conversations between the other pairs is minimized.
The CDMA multiplexing approach is well known and is explained in detail, e.g., in the text “CDMA: Principles of Spread Spectrum Communication,” by Andrew Viterbi, published in 1995 by Addison-Wesley. Basically, in CDMA, the bandwidth of the data to be transmitted (user data) is much less than the bandwidth of the transmission band. Unique “pseudonoise” keys are assigned to each channel in a CDMA transmission band. The pseudonoise keys are selected to mimic Gaussian noise (e.g., “white noise”) and are also chosen to be maximal length sequences in order to reduce interference from other users/channels. One pseudonoise key is used to modulate the user data for a given channel. This modulation is equivalent to assigning a different language to each pair of speakers at a party.
During modulation, the user data is “spread” across the bandwidth of the CDMA band. That is, all of the channels are transmitted at the same time in the same frequency band. This is equivalent to all of the pairs of partygoers speaking at the same time. The introduction of noise and interference from other users during transmission is inevitable (collectively referred to as “noise”). Due to the nature of the pseudonoise key, the noise is greatly reduced during demodulation relative to the user's signal because when a receiver demodulates a selected channel, the data in that channel is “despread” while the noise is not “despread.” Thus, the data is returned to approximately the size of its original bandwidth, while the noise remains spread over the much larger transmission band. The power control for each user can also help to reduce noise from other users. Power control is equivalent to lowering the volume of a loud pair of partygoers.
CDMA has been used commercially in wireless telephone (“cellular”) and in other communications systems. Such cellular systems typically operate at between 800 MHz and 2 GHz, though the individual frequency bands may only be a few MHz wide. An attractive feature of cellular CDMA is the absence of any hard limit to the number of users in a given bandwidth, unlike FDMA and TDMA. The increased number of users in the transmission band merely increases the noise to contend with. However, as a practical matter, there is some threshold at which the “signal-to-noise” ratio becomes unacceptable. This signal-to-noise threshold places real constraints in commercial systems on the number of paying customers and/or data rates that can be supported.
CDMA has also been used in optical communications networks. Such optical CDMA (OCDMA) networks generally employ the same general principles as cellular CDMA. However, unlike cellular CDMA, optical CDMA signals are delivered over an optical network. As an example, a plurality of subscriber stations may be interconnected by a central hub with each subscriber station being connected to the hub by a respective bidirectional optical fiber link. Each subscriber station has a transmitter capable of transmitting optical signals, and each station also has a receiver capable of receiving transmitted signals from all of the various transmitters in the network. The optical hub receives optical signals over optical fiber links from each of the transmitters and transmits optical signals over optical fiber links to all of the receivers. An optical pulse is transmitted to a selected one of a plurality of potential receiving stations by coding the pulse in a manner such that it is detectable by the selected receiving station but not by the other receiving stations. Such coding may be accomplished by dividing each pulse into a plurality of intervals known as “chips”. Each chip may have the logic value “1”, as indicated by relatively large radiation intensity, or may have the logic value “0”, as indicated by a relatively small radiation intensity. The chips comprising each pulse are coded with a particular pattern of logic “1”'s and logic “0”'s that is characteristic to the receiving station or stations that are intended to detect the transmission. Each receiving station is provided with optical receiving equipment capable of regenerating an optical pulse when it receives a pattern of chips coded in accordance with its own unique sequence but cannot regenerate the pulse if the pulse is coded with a different sequence or code.
Alternatively, the optical network utilizes CDMA that is based on optical frequency domain coding and decoding of ultra-short optical pulses. Each of the transmitters includes an optical source for generating the ultra-short optical pulses. The pulses comprise N Fourier components whose phases are coherently related to one another. The frequency intervals around each of the N Fourier components are generally referred to as frequency bins. A “signature” is impressed upon the optical pulses by independently phase shifting the individual Fourier components comprising a given pulse in accordance with a particular code whereby the Fourier components comprising the pulse are each phase shifted a different amount in accordance with the particular code. A class of truly orthogonal codes based on Hadamard matrices is of interest in the context of frequency-phase encoded OCDMA. The encoded pulse is then broadcast to all of or a plurality of the receiving systems in the network. Each receiving system is identified by a unique signature template and detects only the pulses provided with a signature that matches the particular receiving system's template.
The concept of coherent detection of optical signals has a long history and has been pursued as a means of increasing detector sensitivity and selectivity. It is the optical analog of heterodyne (or homodyne) detection in radio technology. In typical coherent detection scenarios, several different data streams each modulate different optical carriers and a single-frequency local oscillator laser is tuned to appropriately align with one of the data-modulated carriers to select that one data stream. In OCDMA systems, the several data streams all modulate the same set of optical carriers and are distinguishable only on the basis of their specific code signatures and not on the basis of the signal frequency component on which they are carried. Past proposals to apply coherently detection techniques to OCDMA signals have relied upon one-by-one coherent detection of each of the N frequencies in the OCDMA signal; that is, after the OCDMA signal has been properly decoded to realign the phases of one data signal, each of the N frequencies is isolated from the composite, individually mixed with its corresponding local oscillator (LO) frequency and detected. As shown in FIG. 1, an optical receiver receives an optical signal and demultiplexes such signal using a demultiplexer 110 in order to obtain the N Fourier components or frequency bins. The demultiplexed signal is decoded using decoder 120. At the detector 100, a comparatively high-power (relative to the received optical signal) local oscillator (LO) laser 130 is frequency-tuned relative to the desired carrier and optically mixed with the several modulated carriers and downshifts the frequency of the desired carrier into electrical passband of an opto-electronic detector. Other carriers fall outside this passband and are hence suppressed in the detection process.
Optical mixing typically occurs in 2×2 optical combiners 150. The received signal and LO signal are the two inputs to each combiner and either of the two output ports can be used to detect the downshifted signal. However, a particularly attractive detector arrangement uses a pair of balanced detectors 160 and 170, one for each output port of the combiner. The outputs of the pairs of balanced detectors for each frequency bin are summed using 180 and 190. The outputs of elements 180 and 190 are subtracted from one another to suppress the common mode signals and effectively double the desired detected signal. The balanced detector pairs yield an electrical signal that is proportional to the product of the electric fields of the incoming signal and LO signal.
Previous receivers required a combiner and a pair of detectors for each frequency bin thereby increasing the complexity of a coherent gated detector. As such, there is a need for a coherent gated detector that reduces the complexity of the detector while increasing the sensitivity of the detector.